ds: cleanup Errno imports and favor constant comparisons

ds: cleanup Errno imports and favor constant comparisons

Stop importing unused constants, and favor integer comparisons
of `$!' over `$!{EFOO}' hash lookups.  Integer comparisons are
slightly faster, even:

Benchmark: timing 30 iterations of cmp_eq, cmp_ne, hash_hit, hash_miss...
    cmp_eq:  1 wallclock secs ( 1.61 usr +  0.00 sys =  1.61 CPU) @ 18.63/s (n=30)
    cmp_ne:  2 wallclock secs ( 1.57 usr +  0.00 sys =  1.57 CPU) @ 19.11/s (n=30)
  hash_hit:  4 wallclock secs ( 3.85 usr +  0.00 sys =  3.85 CPU) @  7.79/s (n=30)
 hash_miss:  4 wallclock secs ( 3.74 usr +  0.00 sys =  3.74 CPU) @  8.02/s (n=30)

	#!/usr/bin/perl -w
	use Benchmark qw(:all);
	use Errno qw(EAGAIN EINTR);
	my ($r, $w);
	pipe($r, $w) or die 'pipe';
	require IO::Handle;
	$r->blocking(0);
	my $buf;
	my $n = 30000;
	timethese(30, {
		hash_hit => sub {
			sysread($r, $buf, 1);
			for (0..$n) {
				next if $!{EAGAIN};
				die 'FAIL';
			}
		}
		,
		'cmp_eq' => sub {
			sysread($r, $buf, 1);
			for (0..$n) {
				next if $! == EAGAIN;
				die 'FAIL';
			}
		},
		hash_miss => sub {
			sysread($r, $buf, 1);
			for (0..$n) {
				die 'FAIL' if $!{EINTR};
			}
		},
		'cmp_ne' => sub {
			sysread($r, $buf, 1);
			for (0..$n) {
				die 'FAIL' if $! == EINTR;
			}
		},
	});